Electrical circuits are used nowadays in a multiplicity of different applications and functions. For operation of the respective electrical circuit, these circuits are usually connected to a voltage source which provides the input voltage necessary for operating the said electrical circuit.
Depending upon the type of electrical circuit and in particular upon the electrical and electronic components employed in the electrical circuit, the correct polarity of the electrical input voltage which the voltage source provides is crucial for the operation of the electrical circuit. In particular in electrical circuits which are operated with a d.c. voltage, if the input voltage is supplied with incorrect polarity to an electrical circuit this may lead not only to a malfunction of the electrical circuit but also to the destruction of electrical or electronic components of the electrical circuit. In order to prevent such a malfunction of the electrical circuit or a destruction of electrical or electronic components, various types of protective circuits exist, which protect an electrical circuit against the consequences of an input voltage with reverse polarity.
Although the present invention is applicable to a large number of electrical circuits and in a large number of fields of use, the present invention is explained in greater detail below with respect to vehicle systems.
In a motor vehicle a reversal of polarity of the input voltage may be caused for example when a battery is changed or when jump-starting. A very simple and cost-effective possibility for protecting an electrical circuit against the consequences of an input voltage with reverse polarity consists of arranging a diode in the pass direction in a supply line of the electrical circuit. A further possibility consists of connecting a rectifier comprising four diodes between the electrical circuit and the voltage source.
However, a disadvantage of the use of diodes as protection against reverse polarity is that there is a power loss in the diodes and a voltage drop occurs on each of the diodes. This leads to a deterioration in the efficiency of the electrical circuit.
In order to reduce the disadvantages of the use of diodes for protection of an electrical circuit against the consequences of an input voltage with reverse polarity, MOSFETs are frequently used which are arranged in the supply branch of the electronic circuit which carries a positive potential with respect to earth. In this case the MOSFET which controls the electrical supply to the electrical circuit is only actuated when the function of the electrical circuit is actually to be used. This means that the electrical circuit is disconnected when its function is not used.
In a vehicle the reversal of polarity of the electrical input voltage usually takes place when a plurality of the electrical circuits installed in the vehicle is not required. Therefore with a conventional protection against reverse polarity based on a MOSFET the electrical circuits of the vehicle are typically deactivated at such a moment. In this way the MOSFET which serves for protection of the electrical circuit is prevented from overloading in the event of a suddenly reversal of polarity of the electrical input voltage by an electrical current flowing out of the electrical circuit, so that this may become incapable of functioning. If the electrical input voltage of the electrical circuit is reversed in polarity with the MOSFET activated, then a very high current would also flow out of the electrical circuit through the MOSFET. If the MOSFET were disconnected at this moment in order to prevent damage to the electrical circuit, the MOSFET would be destroyed because of the high current to be switched.
If MOSFETs are used as protection against reverse polarity, both P-channel MOSFETs and also N-channel MOSFETs can be used.
In the case of a protection against reverse polarity constructed as a P-channel MOSFET, this can be arranged without costly additional wiring in the supply branch of the electronic circuit which is provided for the purpose carrying a positive potential with respect to earth. However, a disadvantage of P-channel MOSFETs is that whilst they have the same power as a comparable N-channel MOSFET they must have significantly larger dimensions and therefore are also substantially more expensive.
On the other hand, if a smaller and thereby more cost-effective N-channel MOSFET transistor is used as protection against reverse polarity, a complex charge pump circuit is necessary in order to raise the potential at the control input of the N-channel MOSFET transistor above the potential at the source terminal of the N-channel MOSFET transistor.
A protection against reverse polarity, with an N-channel MOSFET, which is arranged in the supply branch of the electronic circuit provided to carry an earth potential or a negative potential with respect to earth, could be designed without additional complex electrical wiring. However, such a protection against reverse polarity cannot be used because this supply branch usually also constitutes the earth connection the electronic circuit. This in turn is necessary in order for example to be able to transmit control commands to the electronic circuit and therefore should not be interrupted regularly, as described above.
However, if this supply line is not interrupted regularly, in the event of a suddenly reversal of polarity of the input voltage the electrical circuit of the N-channel MOSFET is overloaded. Overloading of the N-channel MOSFET is possible in particular when the electrical circuit has a high-capacitance smoothing capacitor which is discharged very quickly by a high discharge current when the polarity of the input voltage is reversed.